Beneath the surface: gut microbes, cyathostomins and resident immune cells – Characterising the baseline.

Overview

• This study examines the interactions between cyathostomin parasites, the immune response, and gut microbiota at the mucosal level in horses, revealing important patterns in parasite distribution, immune cell presence, and microbial communities within different regions of the equine large intestine.

Background and Importance

• Cyathostomins: These are the most common and pathogenic gastrointestinal nematodes in horses.
• Life cycle and pathology: Cyathostomins undergo an encystment phase within the large intestine’s mucosa, where up to 90% of the parasite burden resides.
• Clinical effects: Infection severity ranges from chronic protein loss from the gut (protein-losing enteropathy) to acute and sometimes fatal inflammation of the cecum and colon (typhlocolitis).
• Knowledge gap: The complex interactions between cyathostomins, the equine immune system, and gut microbes at the mucosal level were previously underexplored.

Study Objective and Design

• Goal: To characterize baseline relationships among cyathostomin infection, mucosal immune cells, and gut microbiota in different large intestinal regions of horses.
• Sampling: Eleven horses were randomly selected from an abattoir (slaughterhouse) source to represent a broad population.
• Tissue sampling: Three intestinal sites were analyzed: the caecum, right ventral colon, and left dorsal colon.

Parasitological Assessments

• Faecal egg counts (FEC): Measured to estimate parasite load; however, some horses with zero FEC were still infected.
• Worm counts: Included luminal adult worm enumeration and mucosal larval counting.
• Findings:
  • Mucosal larvae were predominantly located in the caecum and right ventral colon.
  • Luminal adult worms were mainly found in the ventral and dorsal colon regions.
  • High larval burdens were associated with deeper tissue invasion, specifically larval penetration into the submucosa, indicating that tissue invasion is density-dependent.

Immunological Analysis

• Techniques: Histopathology and immunohistochemistry to characterize immune cells present in the mucosa.
• Dominant immune cells: T lymphocytes and macrophages were the main immune cells detected in mucosal tissues.
• Other immune responses: Surprisingly, eosinophil counts and goblet cell hyperplasia (which typically correlate with parasite infections) did not correlate with parasite loads in this study.

Microbiota Profiling

• Method: Bioinformatics approaches were used to analyze microbial composition and diversity from sampled regions.
• Regional differences: Microbial communities varied significantly across intestinal locations, with caecal and faecal samples showing the greatest differences.
• Implications: Faecal samples, commonly used to assess gut microbiota, may not accurately reflect the mucosal microbiota in the large intestine.

Key Findings and Implications

• All horses were infected with cyathostomins regardless of their faecal egg counts, indicating that FEC alone is not a reliable infection indicator.
• Parasite burden and location vary regionally within the large intestine: larvae concentrate in some areas while adults prefer others.
• Tissue invasion by larvae correlates with local parasite density, potentially informing disease severity mechanisms.
• T lymphocytes and macrophages dominate the mucosal immune environment, pointing to their key role in responding to cyathostomin infections.
• Traditional markers like eosinophils and goblet cell responses may not be as relevant in all parasite burdens or stages.
• The composition of microbiota differs along the intestinal tract, highlighting complex spatial dynamics in host-microbe-parasite interactions.
• Faecal sampling may be insufficient to capture these complex mucosal bacterial communities.

Contribution to the Field

• This is the first study to characterize the ecological relationships of cyathostomins, immune cells, and microbiota at the mucosal level in the equine large intestine.
• Provides a foundational understanding of spatial dynamics within the gut, which can aid future research on equine gastrointestinal health and parasitology.
• Suggests that targeting specific intestinal regions or immune pathways might improve diagnosis or treatment of cyathostomin-related diseases.